Garnet geochemical compositions of the Bailongshan lithium polymetallic deposit in Xinjiang Province: Implications for magmatic-hydrothermal evolution

Abstract

The Bailongshan deposit is a newly-discovered super-large lithium polymetallic pegmatite deposit. Garnets occur widely in muscovite granites and pegmatites at Bailongshan, but comprehensive study is yet to be done on the geochemical evolution of garnet from the magmatic to the hydrothermal stages. Here, we present a systematic study of in-situ major and trace elemental variations of garnet from Bailongshan, and discuss the geochemical behaviors during magmatic to hydrothermal stage and the mineral indicator of large-scale mineralization of rare metal. Three types of garnet have been identified at Bailongshan, including disseminated garnet in muscovite granite (Grt I), medium-coarse-grained garnet in barren pegmatites (Grt II), and medium-grained euhedral-subhedral garnet in ore-bearing pegmatites (Grt III). EPMA and LA-ICP-MS geochemical analyses reveal that the garnet belong to almandine-spessartine solid-solution series. The garnets have varied REE contents (∑REE = 3.64～382 ppm), and HREE-enrichment, negative Eu anomalies, and LREE below or close to their detection limits. Grt III has higher Mn and Zn, but lower Fe, Y and HREE than those of Grt I and Grt II. The contents of Li, Zr, Hf, Nb and Ta increase from Grt I to Grt II, but decrease abruptly to Grt III. During the evolution of granitic magma, the increasing magma fractionation and fluid-phase accumulation and the crystallization of zircon lead to the continuous decrease of Y and HREE contents from Grt I to Grt III, and the gradual decrease of Y and HREE contents from core to rim of garnet crystals. In the quartz-albite-spodumene pegmatites, the crystallization of spodumene and columbite-group minerals removes Li, Nb and Ta from the melt, which significantly reduces the contents of these elements in garnet. The crystallization of spessartine-rich garnet is a reflection of a high degree of melt evolution at low pressure which facilitates the formation of LCT type pegmatite deposit. The Zr/Hf and Y/Ho ratios also indicate that magma fractionation degree and fluid activity increase from muscovite granites to barren-pegmatites and ore-bearing pegmatites, The combination of some major (e.g., Fe and Mn) and trace elements (e.g., Li, Ta, Nb, Sc and HREE) signatures, as well as the Zr/Hf, Y/Ho ratios in garnets can indicate the magma fractionation degree and the crystallization of some ore minerals (e.g., spodumene and columbite-group minerals), and may be a potential exploration indicator for rare-metal mineralization.